Oftentimes it is desirable to be able to detect individuals within, for instance, a burning building or enemy combatants or troops behind a wall. Moreover, in some instances police can utilize through-wall sensing systems to be able to detect the presence of wanted individuals from a position outside the building.
Through-wall sensing can be used in military operations in urban terrain, for homeland security, for law enforcement and for fire departments. The need to sense behind walls is clear. It will be appreciated that the details of the mission and types of walls or obstructions dictate the design of the through-wall sensors.
The referenced patent application discloses the use of a directional antenna to project a majority of the energy in a given direction. In one embodiment, the radar is a single frequency radar, operating at 900 MHz, with a YAGI antenna, with 13 dB forward gain. While a YAGI antenna may be utilized in order to reduce back lobes and yet have a readily portable unit, the referenced patent application also discloses a flat panel antenna with conductive elements insulated from a ground plane that may be used to further reduce back lobes and is lighter and more easily transportable.
In many instances, for example when detecting moving objects behind dense walls made of brick or concrete, traditional flat panel antennas may not have adequate performance because a substantial amount of energy is reflected from the first wall surface.
What is therefore required is an improved antenna for a low cost, low power drain, compact unit that can be placed against the outside of a wall that can detect motion of individuals within the building and discriminate against inanimate objects.
More particularly, in the aforementioned patent application, what envisioned was a radar with a relatively large antenna. The antenna originally envisaged for this application was a YAGI antenna that was more than three feet long and had to be mounted on a tripod adjacent the wall through which motion was to be detected.
In order to provide a more compact antenna, a panel-type antenna was devised, which was placed against the wall to steady the antenna.
However, placing the antenna against the wall detuned the antenna and caused backward reflections that in essence swamped the radar's receiver. This required an extremely high dynamic range for the receiver of the radar.
Despite utilizing quad-type panel antennas that provide 12-13 dB of forward gain, the inverted and reflected waves at the antenna/wall interface caused considerable difficulties.